While working at Mondosoft, we used to use ISL Light to help our customers when they had trouble installing or configuring our software. It's a really cool tool that allows a supporter to remote control a PC or server, without the hassle of having to open up firewalls etc.
I even used it for personal use when my family needed help (you have probably experienced the following conversation at some point).
Family member: "Why doesn't X work anymore? It keeps giving me an error message"
Me:"What does the error say?"
Family member: "I don't remember, I clicked OK"
Anyway, I thought of buying ISL for myself, to help out in situations like above, but it was pretty pricey. This weekend, I stumbled upon a support tool with many of the same capabilities, and even a few extras - and all for free!
The tool is called TeamViewer and is available in both free and commercial versions. The free version just has a nag dialog that pops up when you end the session.
The cool thing about this software is that your "victim" doesn't need to install anything. After downloading, the installer lets you run the software without installing. It's really easy to use and the UI is pretty intuitive. The client allows you to chat, transfer files (both ways), lock your victim's PC so they can't fiddle with the mouse and keyboard while you work and a couple of other neat features.
The client also has a presentation mode, allowing you to perform remote demos and things like that.
I think it's a pretty neat tool and I'll definitely be using it a lot from now on.